<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>TODO</title>
  <style>
    div {
      height: 600px;
      width: 300px;
      background-color: #fecc45;
      border-radius: 50%;
      /* 设为300px也不是50%的效果 直呼外行 */
      /* border-radius: 150px; */
    }
  </style>
</head>

<body>
  <div></div>
</body>

</html>